5 2- [1]Since the first synthesis of [Ti( η -P ) ] by Ellis and coworkers , the search for characterizable 5 2decaphosphaferrocene complexes has been a challenge in the field of phosphorus-containing [2] ¯ complexes. In addition, the lower phosphorus-containing complexes with [(RC) P ] ligands n 5-n5(n = 1–3) are of particular interest since η -phospholyl complexes show activity in various [3,4,5]homogeneous catalysis applications. Among the series of ferrocene complexes with ¯ [6] [7][(RC) P ] ligands (n = 0–5) the syntheses of A and E have been benchmarks in the n 5-n[8]development of polyphospholyl complex chemistry.

[9] [10,11,12,13]Other ferrocenes of type B as well as B' and C' were synthesized later using phosphaalkyne as starting material, the missing 1,2,3-triphosphaferrocene C was only recently synthesized by Scherer and coworkers by treating the tetraphosphabicyclobutadiene complex 1 1 [14] [{Cp'''(CO) Fe} ( μ,η : η -P )] with diphenylacetylene.2 2 4[15,16]An electrochemical investigation of the polyphosphaferrocenes shows that the replacement of CR fragments by phosphorus atoms in the phospholyl ring enhances the electron richness and thus increases the electron density at the iron center. This modification made the phosphaferrocenes more difficult to oxidize. Unlike the simple metallocene analogues, the complexes of phosphaferrocene have ligating potential to transition metal centers with the ring phosphorus lone-pair electrons not only in π-1 2ligation but also in η -or η -metal ligation. 1